This invention relates in general to determining cross-talk, and more particularly to determining and compensating near-end cross talk.
Personal computers, workstations and printers are common equipment in an office. This equipment is normally connected to one another in a Local Area Network (LAN) to allow communication of information. The physical connections in a LAN system are usually implemented using LAN cables containing copper conductors inside them. Typically, there are eight copper conductors in each LAN cable, with every two copper conductors forming a twisted pair (i.e. four twisted pairs). Data signals are transmitted in each twisted pair of copper conductors. Each LAN cable is terminated in a “RJ-45” connector, in compliance with industry standards.
Each twisted pair of conductors in the LAN cable functions as a separate communication channel. Therefore, data signals in one twisted pair of conductors should not interfere with or induce any signals in another twisted pair of conductors in the same LAN cable.
Various parameters, like attenuation, return loss and Near End Cross-talk (NEXT), can be used to characterize the performance and quality of LAN cables. Cross-talk is a measure of signal isolation between two twisted pairs of conductors in a LAN cable. In order for a LAN system to operate in an acceptable environment, cross-talk isolation in LAN cables should be maintained above a minimum level.
Telecommunication Industry Association (TIA), an industry working group, has defined a LAN channel configuration. The LAN channel configuration is described in the TIA standard 568-B or ISO 11802 2nd edition, which specify a minimum level of cross-talk isolation over a frequency range of 1 to 250 megahertz. In addition, the TIA and ISO further specify that the NEXT introduced by a RJ-45 connector, which is required to connect a test instrument to a cable under test in order to measure cross-talk, should be excluded from the cross-talk measurement in the LAN channel configuration.
Since a raw cross-talk signal has to be taken with the test instrument connected to the LAN channel configuration by an RJ-45 connector, the raw cross-talk signal includes cross-talk effects introduced by the near end RJ-45 connector. Furthermore, the cross-talk effects from the near end RJ-45 connector can be significantly large. Therefore, the raw cross-talk signal obtained by the test instrument has to be processed to compensate for or reduce the cross-talk effects contributed by the near end RJ-45 connector, in order for cross-talk measurements by the test instrument to be compliant with the TIA standard.
In a prior art method, the cross-talk characteristics of a near end RJ-45 connector is determined and a corresponding near-end cross-talk compensation model is produced. When the raw cross-talk signal is received, the compensation model is applied to the raw cross-talk signal, and a compensated near-end cross-talk signal is provided. However, such a heuristic compensation model is static and reflects the cross-talk characteristics of only a typical RJ-45 connector. The static model does not and cannot accurately model different types of RJ-45 connectors manufactured by different manufacturers. Differences between the static model and characteristics of an actual RJ-45 connector that is used with a test instrument can result in inaccuracies in the cross-talk measurement. Furthermore, the connection between the test instrument and the RJ-45 connector undergoes wear and tear due to repeated plugging and unplugging of the RJ-45 connector from the test instrument. This wear and tear can contribute to additional cross-talk effects, which are not compensated for by the static model, being introduced into the cross-talk measurement.
U.S. Pat. No. 5,532,603 teaches a dynamic method for near-end cross-talk compensation. The patent teaches injecting pulse signals of differing pulse widths into a twisted pair of conductors of a LAN cable, and measuring a raw cross-talk signal induced in another twisted pair of conductors. However, as the signals injected into the conductors are pulse signals, the instrument has to perform characterization in the time domain. Such time domain characterization tends to be susceptible to errors due to environmental effects. Furthermore, in order to ensure that cross-talk due to the near end RJ-45 connector is separable from the raw cross-talk signal, the width of the injected pulses should be small. However, a small pulse width results in a test signal of a low signal power, and this makes the induced cross-talk signal difficult to measure.
U.S. Pat. No. 6,522,152 discloses another method of determining and canceling NEXT contributions of a connector interface. The method includes sending a test signal through a channel and receiving a cross-talk signal or response corresponding to the test signal in the frequency domain. The frequency domain cross-talk signal is converted to a time domain cross-talk signal for further processing. Such further processing includes searching the time domain cross-talk signal for a connector response signature and determining an amplitude and a location of the connector response signature. Then an ideal frequency response corresponding to the connector response signature at the estimated amplitude and location is determined. Finally, the connector response signature is canceled with the ideal frequency response to remove NEXT contributions of the connector interface.